DESCRIPTION

The
getopt()
function parses the command-line arguments. Its arguments
argc
and
argv
are the argument count and array as passed to the
main()
function on program invocation.
An element of argv that starts with '-'
(and is not exactly "-" or "--")
is an option element. The characters of this element
(aside from the initial '-') are option characters. If getopt()
is called repeatedly, it returns successively each of the option characters
from each of the option elements.

If getopt() finds another option character, it returns that
character, updating the external variable optind and a static
variable nextchar so that the next call to getopt() can
resume the scan with the following option character or
argv-element.

If there are no more option characters, getopt() returns -1.
Then optind is the index in argv of the first
argv-element that is not an option.

optstring
is a string containing the legitimate option characters. If such a
character is followed by a colon, the option requires an argument, so
getopt() places a pointer to the following text in the same
argv-element, or the text of the following argv-element, in
optarg.
Two colons mean an option takes
an optional arg; if there is text in the current argv-element,
it is returned in optarg, otherwise optarg is set to zero.
This is a GNU extension. If
optstring
contains
W
followed by a semicolon, then
-W foo
is treated as the long option
--foo.
(The
-W
option is reserved by POSIX.2 for implementation extensions.)
This behaviour is a GNU extension, not available with libraries before
GNU libc 2.

By default, getopt() permutes the contents of argv as it
scans, so that eventually all the non-options are at the end. Two
other modes are also implemented. If the first character of
optstring is '+' or the environment variable POSIXLY_CORRECT is
set, then option processing stops as soon as a non-option argument is
encountered. If the first character of optstring is '-', then
each non-option argv-element is handled as if it were the argument of
an option with character code 1. (This is used by programs that were
written to expect options and other argv-elements in any order
and that care about the ordering of the two.)
The special argument "--" forces an end of option-scanning regardless
of the scanning mode.

If getopt() does not recognize an option character, it prints an
error message to stderr, stores the character in optopt, and
returns '?'. The calling program may prevent the error message by
setting opterr to 0.

If getopt() finds an option character in argv that was not
included in optstring, or if it detects a missing option argument,
it returns '?' and sets the external variable optopt to the
actual option character. If the first character
(following any optional '+'or '-' described above)
of optstring
is a colon (':'), then getopt() returns ':' instead of '?' to
indicate a missing option argument. If an error was detected, and
the first character of optstring is not a colon, and
the external variable opterr is non-zero (which is the default),
getopt() prints an error message.

The
getopt_long()
function works like
getopt()
except that it also accepts long options, started out by two dashes.
(If the program accepts only long options, then
optstring
should be specified as an empty string (""), not NULL.)
Long option names may be abbreviated if the abbreviation is
unique or is an exact match for some defined option. A long option
may take a parameter, of the form
--arg=param
or
--arg param.

longopts
is a pointer to the first element of an array of
struct option
declared in
<getopt.h>
as

struct option {
const char *name;
int has_arg;
int *flag;
int val;
};

The meanings of the different fields are:

name

is the name of the long option.

has_arg

is:
no_argument (or 0) if the option does not take an argument;
required_argument (or 1) if the option requires an argument; or
optional_argument (or 2) if the option takes an optional argument.

flag

specifies how results are returned for a long option. If flag
is NULL, then getopt_long() returns val. (For
example, the calling program may set val to the equivalent short
option character.) Otherwise, getopt_long() returns 0, and
flag points to a variable which is set to val if the
option is found, but left unchanged if the option is not found.

val

is the value to return, or to load into the variable pointed
to by flag.

The last element of the array has to be filled with zeroes.

If longindex is not NULL, it
points to a variable which is set to the index of the long option relative to
longopts.

getopt_long_only() is like getopt_long(), but '-' as well
as '--' can indicate a long option. If an option that starts with '-'
(not '--') doesn't match a long option, but does match a short option,
it is parsed as a short option instead.

RETURN VALUE

If an option was successfully found, then
getopt()
returns the option character.
If all command-line options have been parsed, then
getopt()
returns -1.
If
getopt()
encounters an option character that was not in
optstring,
then '?' is returned.
If
getopt()
encounters an option with a missing argument,
then the return value depends on the first character in
optstring:
if it is ':', then ':' is returned; otherwise '?' is returned.

getopt_long() and getopt_long_only() also return the option
character when a short option is recognized. For a long option, they
return val if flag is NULL, and 0 otherwise. Error
and -1 returns are the same as for getopt(), plus '?' for an
ambiguous match or an extraneous parameter.

ENVIRONMENT VARIABLES

POSIXLY_CORRECT

If this is set, then option processing stops as soon as a non-option
argument is encountered.

_<PID>_GNU_nonoption_argv_flags_

This variable was used by
bash
2.0 to communicate to GNU libc which arguments are the results of
wildcard expansion and so should not be considered as options. This
behaviour was removed in
bash
version 2.01, but the support remains in GNU libc.

EXAMPLE

The following example program illustrates the use of
getopt_long()
with most of its features.

BUGS

The POSIX.2 specification of
getopt()
has a technical error described in POSIX.2 Interpretation 150. The GNU
implementation (and probably all other implementations) implements the
correct behaviour rather than that specified.

CONFORMING TO

getopt():

POSIX.2, provided the environment variable POSIXLY_CORRECT is set.
Otherwise, the elements of argv aren't really const, because we
permute them. We pretend they're const in the prototype to be
compatible with other systems.

On some older implementations,
getopt()
was declared in
<stdio.h>.
SUSv1 permitted the declaration to appear in either
<unistd.h>
or
<stdio.h>.
SUSv3 marked the use if
<stdio.h>
for this purpose as LEGACY.
SUSv3 (POSIX.1-2001) does not allow the declaration to appear in
<stdio.h>.